The invention relates to a synchronous demodulation circuit for a carrier which is amplitude-modulated by a video signal, the circuit comprising a series arrangement of a high-frequency amplifier and a demodulator, an output of the demodulator, for supplying a demodulated video signal containing the d.c. voltage component, being connected to a control input of the high-frequency amplifier via a keyed automatic gain control circuit, the demodulator being connected via a variable phase shifter to a circuit input terminal to which a reference carrier is applied.
Such a synchronous demodulation circuit is disclosed in U.S. Pat. No. 3,925,608. In said patent in the construction of the demodulation circuit, the keyed automatic gain control circuit, which is operative in portions of line blanking periods, results in a demodulated video signal having a constant amplitude and a black level located on a reference potential. All this is of particular importance when the received, modulated carrier signal is supplied by a delay device which is beset with a temperature-dependent signal attenuation and phase variation of the carrier. As a field of application, said patent mentions, by way of example, that what is commonly referred to as vertical aperture correction, the video signal to be corrected being subjected after modulation to delays equal to one and to two line periods, it being a condition that the demodulated video signals must only be delayed and not exhibit distortion.
The patent describes that for optimally performing the synchronous demodulation, the reference carrier is supplied via the phase shifter which is adjustable between plus and minus one carrier period. At optimum synchronous demodulation, the demodulator produces the maximum output signal. The automatic gain control circuit is then operative for keeping the amplitude of the demodulation circuit output signal constant. If the demodulation at the demodulator is not optimum, the automatic gain control circuit will cause, for the period of time it is operative within its control range, the demodulation circuit to supply the output signal with the constant amplitude. The automatic gain control circuit is then operative to correct the non-optimum demodulation. If, however, the automatic gain control circuit reaches the upper limit of the control range with the maximum control, then, when the upper limit is exceeded, the amplitude of the output signal can no longer be kept constant and the black level is no longer at the reference potential, which is impermissible.
It has been found that in practice there are delay devices in which the phase of the modulated carrier varies to a great extent in response to temperature variations. When the synchronous demodulation circuit is put into operation and an optimum demodulation is present, this demodulation may vary to such an extent due to the temperature-dependent phase variation at the received modulated carrier that the automatic gain control circuit is adjusted to beyond its control range. Furthermore, it may be possible that when the circuit is put into operation, there was already no optimum demodulation causing the automatic gain control to go beyond its control range already at smaller temperature variations at the delay device. It is alternatively possible that when the demodulation circuit is put into operation, the automatic gain control circuit cannot come into its control range owing to an excessive phase error in the demodulation.